2. VENTS
We have now to consider the external forms, internal contents and distribution of the vents from which the material of the plateaux was discharged. In the Carboniferous system these interesting relics of former volcanoes are far more distinctly defined and better preserved than in older geological formations. Moreover, in Scotland, they are laid bare to greater advantage, both inland and along the sea-coast, and may indeed be studied there as typical illustrations of this kind of geological structure.
Fig. 123.—View of the two necks Dumgoyn and Dumfoyn, Stirlingshire, taken from the south.
These two necks form a conspicuous feature in front of and below the lava plateau, a portion of which is shown on the right hand. The ground-plan of the same necks is shown in [Fig. 124].
Fig. 124.—Ground-plan of Plateau-vents near Strathblane, Stirlingshire, on the scale of 6 inches to a mile.
In external form the necks connected both with the plateaux and the puys generally rise from the surrounding ground as isolated, rounded, conical or dome-shaped prominences, their details of contour depending mainly upon the materials of which they consist. When these materials are of agglomerate, tuff or other readily disintegrated rock, the surface of the domes is generally smooth and grass-covered. Where, on the other hand, they consist wholly or in part of dolerite, basalt, diabase, andesite, trachyte or other crystalline rock, they present more irregular rocky outlines. Illustrations of some of those varying forms are given in Figs. [23] and [123]. In rare instances the vent is marked at the surface not by a hill but by a hollow, as in the great neck in the heart of the Campsie Fells ([Fig. 128]).
Fig. 125.—Ground-plans of double and triple necks in the Plateau series, on the scale of 6 inches to a mile.
A. Barwood Hill and Ravenscraig, east of Dumbarton, double vent. B. The Knock Hill, Largs, Ayrshire, double vent (see [Fig. 23]). C. Dumbowie and Dumbuck, east of Dumbarton, triple vent.
As regards their ground-plan, which affords a cross-section of the original volcanic funnel, the plateau-vents present considerable variety. The simplest cases are those in which the form is approximately circular or somewhat elliptical. Here the outline corresponds to the cross-section of a single and normal orifice. Some examples of this simple type are given in [Fig. 124], which represents a group of vents on the edge of the Clyde plateau near Strathblane. The two larger necks here shown are the same which appear in the view in [Fig. 123].[434] Where two vents have been successively opened close to each other, or where the same vent has shifted its position, the ground-plan may be greatly modified. In some instances the double funnel can be distinctly traced. Thus in the conspicuous Knock Hill above Largs in Ayrshire ([Fig. 125, B]) there are two conjoined necks, and such appears to be also the structure shown by the ground-plan of the neck of Barwood Hill and Raven's Craig, east of Dumbarton ([Fig. 125, A]).[435] But more complex forms occur which point to a still larger number of coalescing necks. A group of hills to the east of Dumbarton gives the ground-plan shown in [C, Fig. 125], where traces may be detected of three separate vents. Still more irregular are long narrow dyke-like masses of tuff or agglomerate which have probably risen along lines of fissure ([Fig. 22, No. 1]). The most striking example of these, however, occur in association with the puys and will be described in later pages.
[434] The illustrations in Figs. [124] and [125] are taken from the field-maps of the Geological Survey on the scale of 6 inches to a mile. The ground represented in [Fig. 124] was mapped by Mr. R. L. Jack.
[435] These ground-plans are likewise taken from the field-maps of the Geological Survey. A and C were mapped by Mr. Jack, B by myself. The shaded parts are intrusive andesites and dolerites; the dark bars in A and C being dolerite dykes of much later date than the necks. The dotted portions mark tuff and agglomerate.
Connected with their ground-plan is the relative size of the plateau-vents. On the whole they are larger than those of the puy series. The simple circular or elliptical type presents the smallest necks, some of them not exceeding 100 feet in diameter. The more complex forms are generally also of larger dimensions. By much the largest vent or connected group of vents is that which lies among the uplands of Misty Law in the heart of the Renfrewshire part of the Clyde plateau, where a connected mass of tuff and agglomerate now occupies a space of about 4 miles in length by 2½ miles in breadth ([Fig. 129]). It has not been found possible, however, to trace the boundaries of the separate vents of this tract, nor to distinguish the material of the necks from that which surrounds them. Another large mass which from its shape may be conjectured to represent more than one vent is the great tract north of Melrose, which measures 8800 by 4200 feet.[436]
[436] The following measurements are, like those in the text, taken from the field-maps of the Geological Survey. Carewood Rig, on the borders of Roxburghshire and Dumfriesshire, 7000 × 2400 feet; the great vent in the middle of the Campsie Fells, 5200 × 2600; Black Law, between Bedrule and Jedburgh, 3400 × 1600; Dumgoyn, Strathblane, 2300 × 1300; Rubers Law, 1500 × 1000; Minto Hill (south), 2300 × 1650; Minto Hill (north), 1500 × 1100; Doughnot Hill, Kilpatrick range, 1000 × 700; four of the smallest agglomerate vents along the northern escarpment of the Clyde plateau between Strathblane and Fintry, 500 × 450, 450 × 400, 250 × 100, 200 × 200; Pike Law, Arkleton, Tarras Water, 500 × 500; Harwood, Stonedge, 5 miles S.E. from Hawick, 500 × 300; Arkleton Burn, Dumfriesshire, 400 × 100; Dalbate Burn, 250 × 120.
The distribution of the necks can best be understood from the maps of the Geological Survey, where they have been carefully indicated. As might have been expected, they are not found outside the original limits within which it may be reasonably inferred that the lavas and tuffs were erupted. They occur most abundantly and attain their largest size in and around the districts where the plateaux are most extensively developed. No doubt a large number of them are concealed under these plateaux. A few appear at the surface among the lavas and tuffs, but by far the largest number now visible have been revealed by denudation, the escarpments having been cut back so as to lay bare the underlying rocks through which the necks rise. Thus, along the flanks of the great escarpment that extends from near Stirling by Fintry and Strathblane to Dumbarton, more than two dozen of agglomerate necks may be counted in a distance of about sixteen miles, while if the necks of lava-form material are included, the number of vents must be about fifty. Nowhere in Scotland do such necks form a more conspicuous feature in the scenery as well as the geology than they do between Fintry and Strathblane, where, standing out as bold isolated hills in front of the escarpments, their conical and rounded outlines present a striking contrast to the terraced escarpments behind them. I would especially refer again to the two remarkable cones of Dumfoyn and Dumgoyn above Strathblane (Figs. [123], [124], [127]). Along the west front of the hills between Gourock and Ardrossan seventeen agglomerate-vents occur in a distance of sixteen miles. In Roxburghshire a group of large agglomerate-necks is dotted over the Silurian country around Melrose and Selkirk[437] (see [Fig. 130]).
[437] In this region and farther southward, besides the plateau-eruptions, a later group of puys is to be seen, and it is difficult to discriminate between the necks belonging to the two groups. Those which lie to the east are probably connected with the plateaux, those to the west with the puys. The latter are referred to on [p. 475].
Fig. 126.—Ground-plan of tuff-neck, shore east of Dunbar.
The surrounding rocks are sandstones, which are much hardened round the vent in the zone marked by the short divergent lines. The arrows mark the direction of dip. See "Geology of East Lothian," Mem. Geol. Survey, p. 44.
From the evidence of these necks it is plain that the volcanic materials of the plateaux must in each case have been supplied not from great central orifices, but from abundant vents standing sometimes singly, with intervening spaces of several miles, often in groups of four or five within a single square mile.
In the interior of the country, it is seldom possible to examine the actual junction of necks with the rocks through which they rise, the boundary-line being usually obscured by debris or herbage. On the coast, the vents of the plateaux have not been bared by the sea so fully as in the case of the much younger series of the east of Fife to be described in later pages. But where the East Lothian plateau touches the shore, the waves have laid bare a number of its minor vents, which have thus been dissected in ground-plan on the beach. As an illustration of these vents an example is given in [Fig. 126], from the shore east of Dunbar. Here the sandstones, which are inclined in an easterly direction at 20° to 25°, are pierced by an irregular mass of tuff. It is observable that in this instance long tongue-like projections of the sandstones protrude into the neck; more frequently the material of a neck sends veins or dykes into the surrounding walls. A volcanic chimney would seem to have been often much shattered and fissured in the course of the volcanic explosions, and the fragmentary material has fallen or been injected into the rents thus caused. As a rule, the rocks immediately around the Carboniferous necks are more or less indurated, as in this instance from the Dunbar shore.
The materials which have filled up the vents connected with the plateau-eruptions generally consist of (a) agglomerates or tuffs, but occasionally of (b) some kind of lava, and frequently (c) of both these kinds of rock combined.
(a) Necks of Agglomerate or Tuff.—These materials vary greatly in the nature and relative proportions of their constituents. Usually the included blocks and lapilli are pieces of andesite, diabase, basalt or other lava, like the rocks of the plateaux. But with these occur also fragments probably detached from the sides of the funnels through which the explosions took place, such as pieces of greywacke, sandstone, limestone and shale. Considerable induration may be observed among these non-volcanic ingredients. In some cases, as in that of the occurrence of pieces of granite referred to on [p. 382], the stones have probably been brought up from some considerable depth. In others it is easy to see that the blocks have slipped down from some higher group of strata now removed from the surrounding surface by denudation. Some striking illustrations of this feature will be cited from necks of the puy-series in the south of Roxburghshire ([p. 476]).
The lava blocks in the tuffs and agglomerates are usually rounded or subangular. Pear-shaped blocks, or flattened discs, or hollow spherical balls are hardly ever to be observed, though I have noticed a few examples in the tuffs of Dunbar. A frequent character of the blocks is that of roughly rounded, highly amygdaloidal pieces of lava, the cellular structure being specially developed in the interior, and the cells on the outside being often much drawn out round the circumference of the mass. Such blocks were probably torn from the cavernous, partially consolidated, or at least rather viscous, top of a lava column. Most of the stones, however, suggest that they were produced by the explosion of already solidified lava, and were somewhat rounded by attrition in their ascent and descent. The vents filled with such materials must have been the scene of prolonged and intermittent activity; successive paroxysms resulting in the clearing out of the hardened lava column in the throat of the volcano, and in the rise of fresh lava, with abundant ejection of dust and lapilli.
Fig. 127.—Section across the vents Dumgoyn and Dumfoyn, and the edge of the Clyde plateau above Strathblane, Stirlingshire.
1. Upper Old Red Sandstone; 2. Shales, cement-stones and sandstones ("Ballagan beds"); 3. White sandstone; 4. Andesite lavas; 5. Agglomerate (shown by the dotted portions), traversed by intrusive diabase. f, Fault. D. Late dolerite dyke.
Necks formed entirely of agglomerate are abundant among the vents connected with the plateaux. As examples of them I may refer to the series already mentioned as fronting the escarpment of the Clyde plateau from Fintry to Largs. Another interesting group rises through the Silurian and Old Red Sandstone rocks to the west of the escarpment of the Berwickshire plateau, that near Melrose forming one of the largest in Scotland.
Fig. 128.—Section through the large vent of the Campsie Hills.
1. Andesite lavas; 2. Agglomerate and tuff; 3. Trachytic and andesitic intrusive rocks.
Fig. 129.—Diagrammatic section across the central vent of the Clyde plateau in Renfrewshire.
1. Andesite lavas; 2. Agglomerates and fine tuffs often much altered; 3. Dykes of trachytic and andesitic rocks; 4. Later dykes of dolerite and basalt.
Illustrations of the varying structure of these vents are given in the accompanying figures. In [Fig. 127], a section is drawn through the two necks Dumgoyn and Dumfoyn, which have already been shown in outline and in ground-plan. The relation of these two vents to the neighbouring plateau to the right can here be seen. [Fig. 128] gives a section taken through the great vent of the Campsie Hills, with the minor adjacent necks of Dungoil, Bin Bairn, and the Meikle Bin.
The diagram in [Fig. 129] is meant to convey in a general way what appears to be the structure of the central vent of the Renfrewshire plateaux, to be afterwards referred to. But, as already mentioned, the limits of the various rocks are too much obscured to allow an accurate delineation to be given of their areas and relations to each other. The Berwickshire plateau supplies abundant interesting examples of tuff necks which rise through the Old Red Sandstone many miles distant from the edge of the lavas. This structure is shown in [Fig. 130].
Fig. 130.—Section across Southern Berwickshire to show the relation of the volcanic plateau to the vents lying south from it.
1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The volcanic plateau; 4. Agglomerate and tuff of the vents; 5. Basalt and dolerite; 6. Lower Carboniferous strata.
Indications may occasionally be observed of an agglomerate vent having been first occupied by one kind of material and then, after being in great measure cleared out by explosions, having been subsequently filled up with another. As an example of this structure I may cite again the double neck of the Knock Hill a little to the north of Largs, of which the outline is shown in [Fig. 23], and the ground-plan in [Fig. 125, B]. This hill rises from the red sandstone slopes that front the great Ayrshire plateau and forms a conspicuous cone the top of which is rather more than 700 feet above the sea. Its summit commands a remarkably extensive and interesting panorama of the scenery of the Clyde, but to the geologist perhaps the most striking feature in the landscape is the range of terraced hills behind, mounting up into the great vents of the Renfrewshire uplands. On these declivities the successive lava-streams that have built up the plateau can be seen piled over each other for a thickness of more than 1000 feet, and presenting their escarpments as parallel lines of brown crag with green slopes between.
The Knock has had its upper part artificially dressed, for lines of trench have been cut out of its rocks by some early race that converted the summit of the hill into a strongly intrenched camp. From the apex of the cone the ground falls rapidly westward into a hollow, beyond which rises a lower rounded ridge of similar materials. It is possible that this western ridge may really form part of the main hill, but the grass-covered ground does not afford sufficient exposures of the rocks to settle this point. From the contours of the surface, it may be inferred that there are two closely adjacent vents, and that the western and lower eminence is the older of the two. This hill or ridge consists partly of a coarse agglomerate, and partly of veins and irregular protrusions of a dark, compact, slightly cellular lava. The stones in the fragmental rock are different olivine-basalts, or other basic lavas, and sandstones. The paste is rough, loose and granular. The sandstone fragments are much indurated and sometimes bleached.
The Knock itself is formed mainly of a remarkably coarse and strikingly volcanic agglomerate. Round the outside, and particularly on the south-east, the rock is finer in texture, compact, and gravelly, or like a mudstone, with few or no imbedded blocks, dull-green to red in colour, and breaking with a clean fracture which shows angular lapilli of various basalts or diabases. At the southern end of the neck, where the surrounding red sandstone can be seen within a few feet of the tuff, the latter is bright red in colour, and contains much debris of red sandstone and marl. Possibly this finer tuff, which is traceable as an irregular band round the outside of the neck, may mark an older infilling of the vent than the agglomerate of the centre; but there is no sharp line to be drawn between the two, though a hollow can sometimes be traced on the surface where they join.
The agglomerate of this locality is one of the most characteristic among the plateau-necks of the Clyde region. Its blocks sometimes measure from two to three feet in diameter. They consist almost wholly of a dark crystalline porphyritic olivine-basalt. These blocks are subangular in form, often with clean-fractured surfaces. Though occasionally slightly cellular, they are never slaggy so far as I could see, nor are any true scoriæ to be noticed among them. The blocks suggest that they were derived from the disruption of an already solidified mass of lava. The agglomerate is entirely without any trace of stratification.
Through this tumultuous accumulation of volcanic debris some irregular veins of olivine-basalt, sometimes glassy in structure, have been injected, and reach nearly to the summit of the hill. This intrusive material resembles generally some of the dark intrusive masses in the Dumbartonshire necks. Like these, it exhibits a tendency to assume a more or less distinctly columnar structure, its columns having the same characteristic wavy sides and irregular curvature. The intrusive rocks in the two eminences of the Knock may be paralleled among the stones in the agglomerate. The neck on its north-eastern side rises steeply from the red sandstones which it pierces, but which, although they are much jointed and broken, are not sensibly indurated. Unfortunately the actual junction of the igneous and sedimentary rocks is concealed under herbage.
Fig. 131.—Section of south end of Dumbuck Hill. East of Dumbarton.
As a rule, the fragmental materials of the plateau-necks are quite unstratified. Their included blocks, distributed irregularly through the mass, have evidently undergone little or no assortment after they fell back into the vents. Occasionally, however, a more or less distinct bedding of the agglomerate or tuff may be observed, the layers having a tendency to dip inward into the centre. One of the most conspicuous examples of this structure is to be found in the hill of Dumbuck, to the east of Dumbarton. This neck, which forms so prominent a feature in the landscape, presents a precipitous face towards the south, and allows the disposition of its component materials to be there seen. The agglomerate consists of a succession of rudely stratified beds of coarser and finer detritus, which on both sides are inclined towards the centre, where a plug of fine-grained olivine-basalt has risen and spread out into a columnar sheet above ([Fig. 131]). In general form this basalt resembles such intrusions as that of Largo Law, to be afterwards described ([Fig. 226]), where what may have been the hollow or bottom of the crater is filled with basalt.
Fig. 132.—Section across the East Lothian plateau to show the relative position of one of the necks.
1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs with a seam of limestone (l); 3. Band of basic sheets at the base of the lavas; 4. Trachytes; 5. Phonolite neck.
(b) Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive Rock.—When the vents have been filled by the uprise of some molten rock, it is generally, as we have seen, of a more acid character than the ordinary lavas of the plateaux. Frequently it consists of some variety of trachyte or andesite, commonly of a dull yellow or grey tint and waxy lustre. Good examples may be seen among the remarkable group of necks on either side of the valley north of the village of Strathblane and in those above Bowling. The three great necks in East Lothian, already alluded to,—Traprain Law (Figs. [132], [133]), North Berwick Law ([Fig. 109]), and the Bass Rock ([Fig. 110])—are masses of phonolite and trachyte, obviously related to the trachytes of the adjacent plateau. A smaller but very perfect instance of a vent similarly filled is to be seen in the same neighbourhood on the shore to the east of North Berwick Law.[438]
[438] See "Geology of East Lothian," Geological Survey Memoir, p. 40.
Examples occur where the funnels of eruption have been finally sealed up by the rise of more basic material, and this has happened even in a district where most of the lava-form necks consist of trachyte or some other intermediate lava. Thus, in the Campsie Fells, several such bosses appear, of which the most conspicuous forms the hill of Dungoil (1396 feet,[ Fig. 128]). Further west, among the Kilpatrick Hills, bosses of this kind are still more numerous. The group of bosses near Ancrum and Jedburgh is mainly made up of olivine-dolerites and olivine-basalts ([Fig. 130]). This more basic composition of itself suggests that these bosses may be connected rather with the puy- than with the plateau-eruptions.
(c) Necks of Composite Character.—In not a few examples, the vents have been filled with agglomerate which has been pierced by a plug or veins of lava-form material. Many illustrations of this composite structure may be observed along the west front of the great escarpments from Fintry to Ardrossan (see Figs. [124], [125], [127] and [128]). In that region the intruded rock is often a dull yellowish or grey trachytic or andesitic material. Olivine-basalt is the chief rock intruded in the vents in the Dumbarton district. Among the Roxburghshire vents, where the injected material is commonly olivine-basalt or dolerite, it occasionally happens, as in Rubers Law, that the uprise of the lava has almost entirely cleared out or concealed the agglomerate, and in some of the bosses, where no agglomerate is now to be seen, the basalt may have taken its place ([Fig. 130]).
The largest and most interesting vents connected with this type of Carboniferous volcano, are those which occur within the limits of the plateaux, where they are still surrounded with lavas and tuffs that probably came out of them. Of these by far the most extensive and remarkable lies among the high moorlands of Renfrewshire between Largs and Lochwinnoch, where the ground rises to more than 1700 feet above the sea (see [Fig. 129]). This area, as already remarked, is unfortunately much obscured with drift and peat, so that the limits of its rocks cannot be so satisfactorily traced as might be desired. I think it probable that several successive vents have here been opened close to each other, but their erupted ashes probably cannot be distinguished. Over a space measuring about four miles in length by two and a half in breadth, the rocks exposed at the surface are fine tuffs, breccias and coarse agglomerates, largely made up of trachytic, andesitic or felsitic material, and pierced by innumerable protrusions of various andesitic, trachytic or felsitic rocks in bosses and veins, as well as also by dykes of a more basic kind, such as dolerites and basalts. Some of the tuffs present a curiously indurated condition; and they are frequently much decayed at the surface.[439] Another large mass of tuff and agglomerate lies a little to the south-west of the main area.
[439] This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge of the Geological Survey of Queensland. See Sheet 31, Geological Survey of Scotland.
After the explosions ceased, by which the vents were opened and the cones of debris were heaped up, heated vapours would in many cases, as in modern volcanoes, continue for a long while to ascend in the funnels. The experiments of Daubrée on the effects of water and vapour upon silicates under great pressure and at a low red heat, have shown how great may be the lithological changes thereby superinduced. It is improbable that where a mass of tuff and lava, lying deep within a volcanic vent, was thoroughly permeated with constantly ascending heated vapours, it should escape some kind of change. I am inclined to attribute to this cause the frequent conversion of the sandstones round the walls of the vents into quartzite. The most remarkable example of metamorphism within a vent which I have observed among the plateaux, occurs in the heart of the Campsie Fells, where, instead of forming a prominence, the neck is marked by a great hollow, measuring about a mile in length and half a mile in breadth ([Fig. 128]).[440] It is occupied mainly by a coarse tumultuous agglomerate, like that of other necks in the same district, but with a matrix rather more indurated, and assuming in certain parts a crystalline texture, so as to be at first sight hardly distinguishable from some of the surrounding andesites. Even in this altered condition, however, its included fragments may be recognized, particularly blocks of sandstone which have been hardened into quartzite. Numerous small veins of pink and yellow trachyte traverse the agglomerate, and are found also cutting the bedded andesites that encircle it.
[440] See Explanation to Sheet 31, Geological Survey of Scotland, par. 21 (1878).
Fig. 133.—View of Traprain Law from the south, a phonolite neck of the Garleton Plateau.